1. Field of the Invention
The present invention relates to nanocomposite dielectrics of carbon nanotubes, high energy density capacitors of carbon nanotubes, and methods for increasing the dielectric constant of a polymer matrix with nanocomposite dielectrics.
2. Description of the Background
Escalating requirements for size efficiency demanded by commercial and military for ground, medical, aircraft and space power systems demand reduced size for components such as capacitors. Power loss of system components impedes size reduction. Future requirements of system size and energy density demand stress capability as well as dielectric constant of film dielectrics to be extended to higher ranges than currently available.
Several parameters are considered important factors for design and fabrication of advanced high energy density capacitors such as dielectric breakdown strength, dielectric constant, and dissipation factor. For high energy density capacitors, as in all capacitors, the total stored energy per unit volume is a function of two key properties of the dielectric, dielectric constant and the dielectric breakdown strength. The total energy density is proportional to the square of the dielectric strength and linearly proportional to the dielectric constant, as shown below in equation 1: Energy Density (ED)=E2∈/8xcfx80 wherein ∈ is the dielectric constant, and E is the operating electric stress.
Power density is proportional to voltage peak energy density times the AC voltage frequency. Therefore, for high energy density capacitors, dissipation factor should be kept to a minimum. One approach for increasing stored energy density of a capacitor is to increase the capability of a dielectric to withstand higher peak voltage stresses. Key material properties for capacitor dielectrics is outlined below in Table 1.
The ultimate energy storage in a capacitor varies by the square of the operating voltage; therefore doubling this voltage gives a four-fold increase in energy storage. However, doubling the capacitance, by doubling the dielectric constant, only give a two-fold increase in energy storage.
Compact, high-temperature and high energy density (HED) capacitors have a myriad of uses in both commercial and military applications. For example, these may be used with domestic utilities and appliances, well-drilling equipment, power supplies, aircraft, satellites, trains, automobiles and medical devices. The high-temperature capability of the capacitors allows electronic devices to be mounted close to aircraft engines. This permits more sophisticated engine actuators, sensors and controls to be implemented with a net reduction in weight achieved through the reduction, or even elimination, of wiring hardware that is necessary when the electronics are remotely located. High-energy density capacitors are also greatly needed for Air Force and Army pulse power applications.
Y. Rao, J. Qu, C. P. Wong, IEEE Trans. on CPMT, 23, 680, December 2000. briefly reviewed the market need for integral decoupling capacitors (a.k.a., embedded capacitance) for hand held devices and computers and predict that by 2004-2006, these applications will require Capacitance per unit area of 20 and 72 nF/cm2 respectively.
The standard relationship between Capacitance C and dielectric constant ∈r is as follows:
C=∈0∈rA/t
where
∈0 dielectric constant of free space (8.854xc3x9710xe2x88x9212 F/m)
∈r dielectric constant of the inculator layer (dimensionless)
A area of the electrical conductor
t thickness of the insulator layer
According to this relationship, the dielectric constant of the insulator layer should be as high as 114 in order to achieve capacitance of 20 nF/cm2. Dielectric constant values as high as 82 for composites comprising lead magnesium niobate-lead titanate in an epoxy matrix have been demonstrated. Filler loadings as high as 80 volume % were required, resulting in composites with poor mechanical properties. Dielectric constants can be predicted for these types of composites using Effective-Medium Theory.
Accordingly, traditional approaches of filling high dielectric constant fillers into a polymer matrix require 80 volume % filler loading to achieve a composite dielectric constant of 82. Thus, polymer-ceramic composites have practical limits.
Furthermore. for miniaturized GPS adaptive antenna array applications, a dielectric constant must be high enough to allow for sufficient size reduction of a patch antenna to fit their physical space requirements, which are four antenna elements need to fit in a 3.5xe2x80x3 square array aperture. If an artificial dielectric with a high enough dielectric constant was commercially available, it would be directly applicable to this application.
(see:http://www.mitre.org/support/papers/tech_papers99xe2x80x9400/rao_characterizing/rao_characterizing.pdf)
Accordingly, a novel nanocomposite dielectric with a high dielectric constant is desired. A high energy density capacitor which can meet commercial and military demands is also particularly desirable.
Accordingly, in a preferred embodiment, the invention provides a nanocomposite dielectric comprising a polymer matrix and a plurality of carbon nanotubes dispersed therein.
In another preferred embodiment, the invention provides a high energy density (HED) capacitor comprising a polymer matrix and a plurality of carbon nanotubes substantially dispersed therein.
In another preferred embodiment, the invention provides a circuit comprising a high energy density (HED) capacitor of the instant invention.
In another preferred embodiment, the invention provides a method for increasing a dielectric constant of a polymer matrix, comprising dispersing a plurality of carbon nanotubes in said polymer matrix to form a nanocomposite dielectric and measuring the dielectric constant of said nanocomposite dielectric.
In another preferred embodiment, the invention provides a laminate comprising a nanocomposite dielectric of the instant invention. Preferably, a metal layer is bonded to at least one side of the dielectric. Preferably, the laminate is incorporated into a multilayer circuit structure to form an embedded capacitor. Preferably, the dielectric is reinforced with glass fabric. Preferably, the dielectric is greater than about 0.002 mm thick.
In another preferred embodiment, the invention provides a mobile antenna comprising a nanocomposite dielectric of the instant invention. Preferably, a dielectric constant of said dielectric increases as size of said antenna decreases.
Preferably, the plurality of carbon nanotubes are substantially single walled carbon nanotubes.
Preferably, the plurality of carbon nanotubes are substantially multi-walled carbon nanotubes.
Preferably, the plurality of carbon nanotubes are a mixture of single walled and multi-walled nanotubes.
Preferably, the polymer matrix is selected from the group consisting of epoxy resins, cyanate ester resins, polyimides, silicones, polybutadiene resins, fluoropolymers, urethanes, acrylics, polycarbonate, polypropylene, polyethylene, polyesters and mixtures thereof.
Preferably, the plurality of carbon nanotubes are oriented parallel to an electric field of the nanocomposite.
Preferably, a metal coating is deposited on the surface of said nanotubes to increase conductivity of said nanotubes.
Preferably, the metallic coating is selected from the group consisting of silver, gold, copper, nickel, aluminum and mixtures thereof.
Preferably, the nanotubes are present at a concentration below a percolation threshold of said nanocomposite dielectric.
Preferably, the nanotubes are mixed with a conductive filler selected from the group consisting of silver particles, nickel coated graphite, metallic coated glass beads, metallic coated hollow glass or ceramic spheres, copper particles, stainless steel fibers, carbon black, gold particles, aluminum particles and mixtures thereof.
Preferably, the nanotubes are mixed with inorganic dielectric particles to increase the volume resistivity of said dielectric.
Preferably, an organic molecule is adsorbed or covalently bonded to a surface of said nanotubes to improve dispersion or increase the volume resistivity of said dielectric.
Preferably, the dielectric has a volume resistivity greater than about 108 ohm-cm. Preferably, the dielectric has a volume resistivity greater than about 1012 ohm-cm.
Preferably, the dielectric has a dielectric constant greater than about 4. Preferably, the dielectric has a dielectric constant greater than about 10. Preferably, the dielectric has a dielectric constant greater than about 40. Preferably, the dielectric has a dielectric constant greater than about 100.
Preferably, the dielectric has a dielectric loss less than about 0.05. Preferably, the dielectric has a dielectric loss less than about 0.02.
Preferably, the dielectric has a dielectric breakdown strength greater than about 1,000 volts/mil. Preferably, the dielectric has a dielectric breakdown strength greater than about 15,000 volts/mil. Preferably, the dielectric has a dielectric breakdown strength greater than about 20,000 volts/mil.shold of said nanocomposite dielectric.
Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the appended claims.